1. Field of the Invention (Technical Field)
The invention described and claimed herein is generally related to methods for improving the adhesion of metals to adhesives, resins, coatings, primers, paints, bonding agents, coupling agents, and other similar adhesive compositions.
More particularly, the present invention is related to methods and compositions which enhance the capability of thermoplastic and thermosetting resins to adhere to metal substrates.
2. Description of the Related Art Including Information Disclosed under 37 C.F.R. .sctn..sctn.1.97.1.99 (Background Art)
Compositions in the nature of adhesives, paints, primers, coatings, bonding agents, coupling agents and the like, are referred to collectively herein as adhesive compositions. It should be understood that, as used herein, the terms "adhesive composition," "adhesive," "adhesive resin," and "resin" are used broadly, referring not only to conventional adhesives used to bond one article to another, but also referring to paints, coatings, primers, bonding agents, coupling agents, and other compositions which are applied to a solid substrate in a liquid or plastic state, and which are subsequently cured or solidified, either partially or wholly, so as to adhere to the substrate.
It will also be understood that any reference herein to a cured adhesive composition, or a hardened or a solidified adhesive composition, is a reference to the adhesive composition in its final state, even though in some cases the adhesive composition in its final state may in fact be only partially hardened or solidified, as for example In an adhesive composition that in its final state is a plastic or elastomeric material.
The adhesive compositions referred to above all depend for their successful application on the formation of a sufficiently strong bond between the adhesive composition and the solid substrate to which it is applied. It has been a continuing effort in this field of art to find ways to improve the strength and consistency of the bond between adhesive compositions and solid substrates. One conventional way to maximize the strength of such bonds is to prepare or treat the surface of the substrate so as to be more receptive to adhesive bonding. Common methods for mechanically treating solid metal surfaces to enhance bonding include cleaning, etching, and roughening the surface of the metal substrate prior to application of the adhesive composition. In the case of copper-clad circuit boards, there has even been resort to growing small crystalline copper dendrites on the surface of the copper to mechanically enhance the bonding thereto of thermosetting resins.
The other primary approach to strengthening such bonds is to resort to adhesive compositions which form chemical bonds between the metal substrate and the adjacent adhesive composition. This has been a more difficult problem because many metal substrates are relatively inert and do not readily react to form chemical bonds with common adhesive compositions. Also, thermoplastic adhesive compositions typically do not undergo any chemical reaction upon hardening by cooling, and thus do not form chemical bonds with a substrate. Some of the thermosetting adhesive compositions, which cure by chemical polymerization reactions, might be candidates for forming chemical bonds by reacting with a substrate, but the polymerization reactions which result in curing of these adhesive compositions are typically not reactions which will cause bonding of the adhesive composition to a solid substrate, particularly a metal. Accordingly, the present invention is addressed to providing compositions and methods which result in chemical bonding between a metal substrate and a conventional adhesive composition.
In some of the applications discussed below the substrate is a solid metal article to which a bulk adhesive or an adhesive coating is to be adhered. However, it will be seen that there is also contemplated herein the application of the present invention to the bonding of metal planal elements, such as sheet, cast, sintered, electroplated, or electroformed surfaces, fibers, wires, metal foils, micro-spheres, or metal particulate or powder components, alone or in a polymeric or resinous matrix, for the purpose of making high-strength reinforced composite materials.
By way of background, adhesives may be characterized by their solidification process. For example, solvent loss adhesives set, or harden, by loss of solvent through evaporation or by dispersion into a surrounding solid substrate to which a mechanical bond is formed.
Hot melt, or thermoplastic, adhesives form a bond when a thermoplastic material is heated above its softening point and is allowed to cool while in contact with a substrate surface. Examples include polyvinyl acetate, polystyrene, polyamides, acrylics, polyethylene, and ethylvinyl acetate.
Chemical reaction adhesive compositions utilize thermosetting synthetic organic materials. These adhesive compositions are caused to polymerize by the introduction of a chemical reactant or a catalyst, or by heating, or by exposure to air. These adhesives are often characterized by high structural strength but do not always bond well to solid substrates, for the reasons mentioned above. Examples include the epoxy resins, polyurethane resins, phenolics, amino resins, polyesters, and isocyanates.
The adhesive bonding of metal-polymer interfaces is becoming increasingly important in manufacturing. Mechanical fasteners are difficult to configure and expensive to apply in many cases. Adhesive bonding is often the preferred method for the manufacture of complex, multi-component structural shapes and has the additional advantage over mechanical fastening in that it can be applied over large areas of contact and thus avoid stress concentration near mechanical fastening points. However, many metals are by their very nature relatively inert and/or stable materials to which it is difficult to form strong adhesive bonds. Most adhesives in use today depend on relatively weak physical forces to achieve adherence to the substrate on which they are used. If a covalent chemical bond can be formed between the surfaces to be bonded, adhesion between the adherends can be greatly enhanced over any physical interaction. Modern adhesives attempt to utilize this covalent chemical bonding to effect a strong bond. The principal problem arising in these attempts is that the adhesive must be carefully tailored to each application or the substrates must undergo special surface treatment to allow their participation in the bonding process. In many cases, both adhesive tailoring and surface treatment are needed to effect the bond.
As will be discussed below, the present invention is based upon the use of certain azides, which can be thermally or photochemically decomposed to nitrenes. (Nitrenes are sometimes referred to in the older literature as azenes, imenes, azylenes or imidogens.) Nitrenes in turn react to form strong covalent bonds with many compounds, including ordinarily inert metal substrates as well as with most organic resins. Azides have been previously used in photoresist applications to render polymer coatings insoluble after exposure to ultraviolet and visible light. This photocrosslinking technique has been applied to photoresist formulations for patterning metals and semiconductors.
It has been previously known to use nitrenes to modify surfaces. For example, U.S. Pat. No. 3,666,536 to Olsen, et al., discloses the process of generating gaseous nitrene species by pyrolytic decomposition of certain nitrogeous precursors, followed by exposing the gaseous nitrenes to a solid material to thereby effect modification of the surface characteristics of the solid material.
It has also been previously known to use azidosilanes as coupling agents in filled polymers. For example, the use of azidosilanes for this purpose is disclosed in the paper by F. J. Kolpak, Applications of Azidosilane Coupling Agents in Reinforced Thermoplastic Composites, SAMPE Quarterly, Vol. 18, No. 1, October 1986, pp 21-27. The method Kolpak is however not suitable in the present invention because the azidosilanes are hydrolytically unstable and react with water to form the free amine and the corresponding sulfonic acids. As another example, U.S. Pat. No. 4,055,701 to Marsden, et al., discloses the use of certain water-soluble, ionic azidosilanes as coupling agents in the glass finishing industry. Marsden discloses coating a solid, such as glass, with the water-soluble, ionic azidosilane and allowing it to dry, followed by applying an organic polymer to the coated surface and curing the polymer at an elevated temperature. One disadvantage however of using a water-soluble azidosilane for this purpose is that the azidosilane, after reaction and incorporation between a solid and a polymer as a bonding agent, is hydrolyzable and the resulting bond is thus susceptible to attack by water.
A series of U. S. Pat. Nos. to J. Brent Thomson (3,697,551; 3,705,911; 3,706,592; 3,715,371; and 3,813,351) discloses certain nitrogen containing silane compounds and their uses in combination with various polymers and solvents as adhesion promoting agents and as priming compositions.
A series of U.S. Patents to James N. Haynes (U.S. Pat. Nos. 3,686,231; 3,814,657; 3,914,262; 3,946,051) discloses certain azidoformates and their use to improve the adhesion of polyester materials to other materials, for example epoxy adhesives. In accordance with the disclosures of Haynes, a solution of an azidoformate is applied to a surface of a solid polyester material, then dried and heated to evaporate the solvent and activate the azidoformate. The surface prepared in this manner is demonstrated to form bonds with conventional adhesives which are of increased strength.
U.S. Pat. No. 3,616,199 to Breslow also relates to improved adhesion to polyester materials. Breslow discloses the use of certain polysulfonyl azides to modify the surface of polyester materials, particularly tire cords, so that they will form stronger bonds to, for example, vulcanized rubber used to manufacture tires.
U.S. Pat. No. 4,099,910 to Herweh discloses the use of fluorinated azidoformates and sulfonyl azides for the purpose of treating fabrics to render them fire resistant. The compounds are applied in solution to a fabric, then heated to evaporate the solvent and activate the azide so as to bond the fluorinated compound to the fabric surface. U.S. Pat. No. 3,997,571 to Buckley also discloses the use of fluorinated azidoformates and fluorinated sulfonyl azides as textile-finishing agents. U.S. Pat. No. 4,309,453 to Reiner, et al., discloses a the use of certain monomeric azide or diazo compounds, having a hydrophilic, hydrophobic, oleophobic and/or ionic group present at the opposite end of the compound from the azide or diazo moiety, for the purpose of altering the surfaces of solid polymeric materials, for example for the purpose of increasing the susceptibility of the solid polymeric materials to printing or adhesion.